Bonded materials are used in a variety of structural applications. For example, adhesively bonded, laminated composite structures are increasingly being used in aircraft construction to reduce weight, reduce or eliminate the number of separate components, and improve fuel efficiency. The presence of material defects in a composite aircraft structure can lead to disastrous failure of the structure under flight loads. These defects may exist in the composite laminate itself, as well as in the adhesive bonds in the structure. The defects may arise as a result of damage during service, or in the original manufacturing process.
The growing ubiquity of composite structures has led to an increased need for techniques to evaluate the strength of the composite structures, including the adhesive bonds themselves, without damaging or destroying the composite structures. Conventional nondestructive evaluation (“NDE”) techniques are useful when a gap, crack, or void is present in a bonded material. However, conventional NDE techniques do not adequately identify deficiencies, such as weak bonds or “kissing bonds,” where materials bonded together are in contact but without adequate structural strength. These deficiencies can result from bond surface contamination, improperly mixed or outdated adhesives, and improper adhesive application.
Laser bond inspection (“LBI”) is an NDE technique for testing the integrity of bonded materials and structures. LBI is a method that involves sending a precisely controlled dynamic stress wave through an adhesive bond of a composite structure. Generally speaking, and with reference to FIG. 1, LBI 100 involves the deposition of laser energy 102 at a first surface 106 of a bonded material 104, generating a compressive stress wave 108. A first laser pulse 102 is applied to first surface 106 of bonded composite structure 104 with an opaque overlay 112 and a transparent (tamping) overlay 110 applied to surface 106. Laser pulse 102 passes through transparent overlay 110 and is absorbed by opaque overlay 112. A plasma is created and as the plasma blows off, compressive stress wave 108 is induced into surface 106. Generally speaking, no intentional heating occurs in the composite structure, and surface damage is attempted to be avoided. The shape of stress wave 108 can be tailored to several hundreds of nanoseconds in duration. The magnitude of stress wave 108 is a function of the laser input irradiance, which facilitates generation of calibrated stress waves. Compressive stress wave 108 propagates through bonded material 104, through a bond of interest 114, to a second surface 116 of bonded material 104, where stress wave 108 is reflected as a tensile wave (not shown). The tensile wave propagates back through bonded material 104 and, when it reaches bond 114, stresses bond 114. The application of dynamic stress on bonded material 104 is selected to be low enough to have little or no effect on the integrity of bonded material 104 or bond 114 if bond 114 is sufficiently strong. However, if bond 114 is below a suitable strength, the tensile wave will cause bond 114 to fail (or will expose its non-bonded nature, in the case of a kissing bond).
By observing changes in the front surface or back surface motion, a determination can be made on the strength and reliability of the bond. Various sensing means exist for observing changes in the surface motion. U.S. Pat. No. 6,848,321 (The Boeing Company), which is incorporated by reference herein in its entirety, teaches the use of Velocity Interferometer System for Any Reflector (VISAR) probes for capturing surface velocity. U.S. Pat. Nos. 7,770,454 and 8,156,811 (LSP Technologies, Inc.), both of which are incorporated herein by reference in their entireties, teach LBI systems using VISAR probes, electromagnetic acoustic transducer (EMAT) coils, capacitance probes, and piezoelectric ultrasonic transducers (UT) as sensors. While various LBI systems employing sensing means exists for measuring surface motion, the LBI systems disclosed herein provide additional advantages in evaluating bonds in angled structures and confined spaces.